This invention relates to a braking device for a power working machine, and in particular to a braking device for braking the revolving power transmission portion of a working portion during the idling of an internal combustion engine for a portable type power working machine, such as a hedge trimmer.
A portable type power working machine such as a hedge trimmer is generally constituted by a prime mover case having a prime mover such as an internal combustion engine or an electric motor therein, amount base portion enclosing a transmission case having a power transmission device such as a centrifugal clutch and gears to be actuated by the prime mover, a working portion comprising clipper blades to be actuated via the aforementioned power transmission device by the prime mover, and a handling portion attached to the prime mover case or the mount base portion. The handling portion is constituted, for the convenience of manipulation thereof, by a front handle portion and a rear handle portion. The rear handle portion is provided with a throttle lever for adjusting the opening degree of a throttle valve where the internal combustion engine is to be employed as a prime mover.
In the use of such a portable type power working machine, both the front handle portion and the rear handle portion are grasped by the user's hands, and the aforementioned throttle lever is rotated with an operator's finger so as to adjust the opening degree of the throttle valve via a throttle cable. The throttle valve is normally biased, as a matter of the usual construction of a carburetor, at the minimum opening degree (idling opening) in which the engine runs at an idling speed. However, since the speed is low, the centrifugal clutch remains disengaged, and the driving power is not transmitted to the working portion, such as clipper blades.
When the throttle valve is rotated to pull the throttle cable over a predetermined length, the play is eliminated and the throttle valve is caused to move from the idling position to a higher speed position. When the rotational speed of a crank shaft of the engine is increased to slightly exceed a predetermined speed, the centrifugal clutch engages and hence the driving power is transmitted to the working portion.
When the throttle lever is released, the throttle valve is caused to return to the idling opening, thereby automatically returning the engine into an idling state, and hence the centrifugal clutch is also disengaged, thus suspending the transmission of driving power to the working portion.
When operator's finger is released from the throttle lever so as to suspend the rotating manipulation of the throttle lever for suspending the work in the conventional portable power working machine, the engine is automatically brought into an idling state and the centrifugal clutch is also brought into a state of cut-off, thus suspending the transmission of driving power to the working portion. However, due to the inertia, the cutting blades of the working portion is still allowed to be actuated for a while.
The movement of the blades of the working portion immediately after the suspension of work may invite unexpected damage or accident. For example, the blades may contact with other matters or a human body at the occasion of carrying the working machine immediately after the work.
In order to overcome these problems, there has been proposed a portable type power working machine wherein a braking means is mounted on the power transmission system between the engine and the working portion (Japanese Patent Publication S/58-54641; U.S. Pat. No. 3,290,871). However the braking means proposed in these references are merely intended to apply a braking to the engine or the working portion when it is required to do so, and are not intended to automatically cut off the power transmission from the engine to the working portion so as to brake the working portion when the engine is shifted to an idling state. Namely, the aforementioned problems cannot be overcome by the braking means proposed in these references.
Furthermore, the braking means proposed in these references are designed such that when the braking is desired to be applied, the brake lever exclusively designed therefor is required to be pulled in the direction to pull a brake band. Namely, the direction of manipulating the brake lever differs from the direction of shifting the engine to an idling state, i.e. the manipulating direction of the throttle lever for moving the throttle valve to the idling opening (generally, the direction of shifting the throttle valve to the idling opening is the direction of releasing the throttle lever). Therefore, the braking means proposed in these references are not suited for solving the aforementioned problems.